Two-terminal linear sensor

ABSTRACT

A magnetic field sensor includes a linear magnetic field sensor to produce a voltage proportional to a sensed magnetic field and an interface having only two terminals for external connections. The two terminals of the interface include a power supply terminal and a ground terminal. The interface includes a voltage-controlled current generating device that is connected between the two terminals and is controlled by the voltage to provide a current signal that is proportional to the sensed magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

This invention relates generally to magnetic field sensors and, inparticular, to linear magnetic field sensors.

BACKGROUND OF THE INVENTION

Magnetic field sensors that generate an output voltage signal that isproportional to a magnetic field are well known. To date, suchconventional linear magnetic field sensors use an output structure thatrequires a separate output connection in addition to power and groundconnections. In an application, each interconnection contributes tooverall cost and space requirements. In low-cost sensor applications,for example, automotive tachometers, magnet actuated alarm systems andthe like, each interconnection may represent a significant cost.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention is directed to a magnetic fieldsensor. The magnetic field sensor includes a linear magnetic fieldsensor to sense a magnetic field and to produce a voltage proportionalto the sensed magnetic field. The magnetic field sensor further includesan interface having only two terminals including a power supply terminaland a ground terminal. The interface includes a voltage-controlledcurrent generating device connected between the two terminals,controllable by the voltage to provide a current that is proportional tothe sensed magnetic field.

Embodiments of the invention may include one or more of the followingfeatures. The linear magnetic field sensor may include a Hall-effectelement or a magnetoresistive (MR) element to sense the magnetic field.The voltage-controlled current generating device may be operable as acurrent sink when the power supply terminal is connected to an externalcurrent sensing device or, alternatively, a current source when theground terminal is connected to an external current sensing device. Thevoltage-controlled current generating device may include an operationalamplifier coupled to a transistor. The linear magnetic field sensor andinterface may be implemented in a single integrated circuit.

In another aspect, the invention is directed to a circuit that includesa linear magnetic field sensor and a current sensing device. The linearmagnetic field sensor comprises an integrated circuit having only twoterminals, a power terminal and a ground terminal. The current sensingdevice, which is connected to the linear magnetic field sensor, isusable to measure a current that is proportional to a magnetic fieldsensed by the linear magnetic field sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention, as well as the invention itselfmay be more fully understood from the following detailed description ofthe drawings, in which:

FIG. 1 shows an exemplary two-terminal linear magnetic field sensor thatincludes a three-terminal linear magnetic field sensor and athree-to-two-terminal interface that includes a voltage-controlledcurrent generating device;

FIGS. 2A-2B show an exemplary current sensing circuit that utilizes thetwo-terminal linear magnetic field sensor (from FIG. 1) and in which thevoltage-controlled current generating device is operable as a currentsink (FIG. 2A) or a current source (FIG. 2B);

FIG. 3 shows one exemplary implementation of the voltage-controlledcurrent generating device (from FIG. 1) that includes an operationalamplifier and a bipolar junction transistor (BJT); and

FIG. 4 shows an example of a three-terminal linear magnetic field sensorfor use in a two-terminal linear magnetic field sensor like that shownin FIG. 1.

Like reference numerals will be used to represent like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a two-terminal transducer 10 that includes atransducer 12 having three terminals is shown. In one exemplaryembodiment, as illustrated in FIG. 1, the transducer 10 is atwo-terminal linear magnetic field sensor (“two-terminal sensor 10”) andthe transducer 12 is a three-terminal linear magnetic field sensor(“linear magnetic field sensor 12” or, simply, “sensor 12”). The term“terminal” as used herein refers to a position or contact point at whichan external electrical connection can be (or is) made.

The sensor 12 operates to sense a magnetic field and to produce anoutput voltage that is proportional to the sensed magnetic field. Morespecifically, the output voltage changes proportionately to change inmagnetic field strength. The output voltage (labeled “Vout”) is providedat a first terminal, output terminal 14. Power is provided to the sensor12 through a second terminal, VCC terminal 16. A connection to ground isprovided to the sensor 12 through a third terminal, GND terminal 18. Inthe illustrated sensor, the VCC terminal 16 is connected to a voltageregulator 20. Thus, the supply voltage provided to VCC terminal 16 is aregulated voltage. In one embodiment, the sensor 12 may be implementedas an integrated circuit (IC), that is, a single chip electroniccircuit. Alternatively, the voltage regulator 20 may be viewed as partof the sensor 12 architecture and thus included to form a sensor 12′.The sensor 12′ may be implemented as an IC as well. In yet anotheralternative implementation, the entire two-terminal sensor 10 may beconstructed as an IC. In a sensor IC, the terminals might correspond todevice pins. If the sensor 12 or 12′ is part of a larger IC, theterminals 14, 16 and 18 would be internal circuit nodes connecting thesensor 12 or 12′ to other circuitry of the two-terminal sensor 10.Hereinafter, references to sensor 12 will be taken to mean sensor 12 orsensor 12′.

Still referring to FIG. 1, the two-terminal sensor 10 also includes athree-to-two-terminal interface 22. The interface 22 includes a firstterminal 24 corresponding to a VCC terminal, which is provided toconnect to an external power supply, and a second terminal 26corresponding to a ground (GND) terminal, which is provided to connectto ground. The interface 22 further includes a voltage-controlledcurrent source or sink 28 referred to more generally herein as avoltage-controlled current generating device 28. The voltage-controlledcurrent generating device 28 is coupled to the GND terminal 26 via aground path 30 and is coupled to the VCC terminal 24 via a supply path32. The voltage-controlled current generating device 28 is also coupledto the output terminal 14 of the sensor 12 by a sensor output line 33.The voltage-controlled current generating device 28 receives the outputvoltage Vout from the sensor 12 on the sensor output line 33. Thevoltage-controlled current generating device 28 provides a current thatis proportional to the sensor output Vout. Consequently, the generatedcurrent is proportional to the sensed magnetic field.

The GND terminal 18 of the sensor 12 is connected by a line 34 to theground path 30 (and therefore the GND terminal 26) at a node 36. Thevoltage regulator 20, or the sensor 12′, is coupled to the supply path32 at a node 38. If a voltage regulator such as voltage regulator 20 isincluded in the two-terminal sensor 10, it may be considered part of theinterface 22 or part of the sensor 12′, as mentioned earlier.

FIGS. 2A and 2B show the two-terminal sensor 10 in a current sensingcircuit. FIG. 2A shows a current sensing circuit 40 configured for highside current sensing, with a current sensing element 42 connectedbetween a power supply (labeled “V+”) and the VCC terminal 24. Thesensor's GND terminal 26 is connected to ground. A bypass capacitor,shown as bypass capacitor 44, may be connected between the two terminals24, 26. FIG. 2B shows a current sensing circuit 50 configured for lowside current sensing, with the current sensing element 42 connectedbetween the sensor's GND terminal 26 and ground. In the circuit 50, theVCC terminal 24 is tied directly to the power supply. A bypasscapacitor, again shown as bypass capacitor 44, may be connected betweenthe two terminals 24, 26. Although not shown, it will be appreciatedthat the current sensing device 42 and the monitored circuit of interest(i.e., the two-terminal sensor 10) could be connected to the supply andground with switches for full-range, bidirectional current sensing.

Referring to FIGS. 2A-2B in conjunction with FIG. 1, thevoltage-controlled current generating device 28 of the two-terminalsensor 10 is employed as a current sink in the circuit 40 (FIG. 2A) andas a current source in the circuit 50 (FIG. 2B). Thus, the currentflowing through the current sensing element 42 is sinked or sourced inproportion to the magnetic field sensed by the sensor 12 (from FIG. 1).A value indicative of the current flowing through the voltage-controlledcurrent generating device 28 may be sensed by sensing the currentthrough a sense resistor. Accordingly, in one embodiment, and asdepicted in FIG. 2B, the current sensing element 42 of circuit 50 may beimplemented with a sense resistor (“Rs”). Although not shown, thecircuit element 42 of circuit 40 (FIG. 2A) may be implemented with asense resistor Rs as well. A sense resistor Rs would develop a voltageproportional to the current flowing from the power supply to thevoltage-controlled current generating device 28 in circuit 40. A senseresistor Rs in circuit 50 would develop a voltage proportional to thecurrent flowing through the voltage-controlled current generating device28 to ground.

Referring to FIG. 3, in one exemplary implementation, thevoltage-controlled current generating device 28 may be designed toinclude a non-inverting operational amplifier (or op-amp) 60 that iscoupled to a pass element 62 such as a transistor, as shown. The passelement 62 can be bipolar, JFET, MOSFET, or a combination. In theillustrated example, the pass element 62 is shown as a bipolar junctiontransistor (BJT). The op-amp 60 has two inputs, a positive input 64 thatreceives Vout (from sensor 12) and a negative input 66, and an output68. The output 68 is coupled to the control terminal (base) of the BJT62. The transistor's input terminal (collector) is connected to supplypath 32. A feedback loop 70 is provided between the transistor's outputterminal (emitter) at node 72 and the negative input 66 of the op-amp60. A resistor (“R”) 74 is connected in series between the node 72 andthe GND path 30.

The op-amp 60 receives from the sensor 12 the voltage proportional tostrength of magnetic field, that is, Vout. The sensor output Vout,provided as an input to the voltage-controlled current generating device28 at op amp input 64, operates as a control voltage for the device 28.Current flows through the transistor 62 as a function of that controlvoltage, i.e., proportional to the control voltage Vout. The op-amp 60adjusts the current to the emitter of the transistor 62 so that itremains equal to Vout/R. When connected in a negative feedbackconfiguration, as shown, the op-amp 60 will try to make the output 68whatever voltage is necessary to make the voltages at inputs 64 and 66as nearly equal as possible. In other words, the op amp feedback loop 70forces the output current flowing through the transistor 62 to beproportional to Vout. For this example implementation, the transferfunction of the two-terminal sensor 10 would be given by Vout/R*Rs.

The two-terminal sensor 10 therefore sources or sinks current directlyproportional to the magnetic field sensed by the internal sensor 12. Thecurrent can be measured by some technique, such as using a currentsensing element like current sensing element 42, e.g., a sense resistor,as illustrated in FIG. 2A-2B, in conjunction with a control device. Whenthe current sensing element is a sense resistor (as shown in FIG. 2B),the control device would be used to measure the voltage across the senseresistor. The control device may be part of or coupled to an applicationmicrocontroller or microprocessor. The control device could beconfigured to continuously monitor the sense resistor.

Potential applications include, for example, control modules thatperform various measurement and/or control functions. The currentsensing element, e.g., a sense resistor, as described above, in anapplication control module would allow for the interpretation of thesensor output using the control device. The control device could be asimple device such as a comparator. In an application, the two-terminalsensor 10 could be used for any type of linear magnetic field sensing,such as current sensing, motor control and position/displacementsensing, to give but a few examples.

The current sensing element 42 can be located near the control devicewhile the two-terminal linear magnetic field sensor 10 is locatedelsewhere in the application. The use of a two-terminal linear magneticfield sensor reduces the number of interconnections required to obtainthe same information available from a three-terminal linear magneticfield sensor. The elimination of an interconnection (which mightrequire, for example, a wire, cable, printed circuit board trace orother interconnecting component) can reduce overall system cost as wellas save space.

Referring to FIG. 4, a simplified example of the three-terminal linearmagnetic field sensor 12 (from FIG. 1) is shown. In the illustratedexample, the sensor 12 is a Hall-effect sensor. It includes a Hallsensing device 80 having at least one Hall-Effect sensing element, aswell as various signal conditioning and peripheral components to makethe sensed signal usable to applications. For example, and as shown inthe figure, the sensor 12 can include dynamic offset cancellationcircuitry 82 (for chopper stabilization), an amplifier stage 84, afilter 86 and an output stage 88 to provide the voltage signal Vout atthe output terminal 14. The sensor 12 may include other features, suchas temperature compensation, internal gain and offset trim adjustment,represented generally by optimization circuitry block 90. Other Hallsensor designs could instead be used. Although a Hall-effect device isillustrated, the design of the internal sensor 12 may be based on anytype of linear magnetic field sensor architecture. Thus, the sensingdevice could also be implemented with, for example, a magnetoresistive(MR) or other type of magnetic field sensing element (or elements). TheMR element may be made from any type of MR device, including, but notlimited to: a anisotropic magnetoresistance (AMR) device; a giantmagnetoresistance (GMR) device, including unpinned sandwich,antiferromagnetic multilayers and spin valve; a magnetic tunnel junction(MTJ, also known as spin-dependent tunneling or “SDT”) device; and atunneling magnetoresistance (TMR) device. The linear magnetic fieldsensor architecture of sensor 12 could be designed to provide either ananalog output signal or its equivalent (for example, an output signalthat digitally encodes analog signal levels, such as a pulse widthmodulation signal).

Although the sensor 12 is described as a “linear” sensor, it will beunderstood that the sensor may exhibit linear and/or non-linearbehavior. If the linear sensor exhibits non-linear behavior, the outputvoltage may be adjusted so that it is proportional to the sensor input,that is, the output voltage provided as Vout 14 is one that has been“linearized”. This linearization may be achieved with appropriate signalconditioning, conversion or other techniques. In the exampleimplementation shown in FIG. 4, the linearization may be handled byblock 90. Nevertheless, it can be said that the sensor 12 produces anoutput voltage that is proportional to the sensed magnetic field.Alternatively, or in addition, it may be desirable to allow the outputvoltage provided as Vout 14 to be a continuous, non-linear output.

Although the embodiments discussed above relate the transducer 10 tomagnetic field sensing, a transducer with a two-terminal interface asdescribed above can be applied to other types of sensing and measurementas well.

All references cited herein are hereby incorporated herein by referencein their entirety.

Having described preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may be used. It is felttherefore that these embodiments should not be limited to disclosedembodiments, but rather should be limited only by the spirit and scopeof the appended claims.

1. A magnetic field sensor, comprising: a linear magnetic field sensorto sense a magnetic field and to produce a voltage proportional to thesensed magnetic field; and an interface coupled to the linear magneticfield sensor and having only two terminals including a power supplyterminal and a ground terminal; wherein the interface includes avoltage-controlled current generating device connected between the twoterminals, controllable by the voltage to provide a current proportionalto the sensed magnetic field.
 2. The magnetic field sensor of claim 1wherein the linear magnetic field sensor comprises a Hall-effect elementto sense the magnetic field.
 3. The magnetic field sensor of claim 1wherein the linear magnetic field sensor comprises a magnetoresistiveelement to sense the magnetic field.
 4. The magnetic field sensor ofclaim 1 wherein the voltage-controlled current generating device isoperable as a voltage-controlled current sink when the power supplyterminal is connected to an external current sensing device.
 5. Themagnetic field sensor of claim 1 wherein the voltage-controlled currentgenerating device is operable as a voltage-controlled current sourcewhen the ground terminal is connected to an external current sensingdevice.
 6. The magnetic field sensor of claim 1 wherein thevoltage-controlled current generating device comprises an operationalamplifier coupled to a transistor.
 7. The magnetic field sensor of claim6 wherein the transistor comprises a bipolar junction transistor.
 8. Themagnetic field sensor of claim 6 wherein the transistor comprises aMOSFET.
 9. The magnetic field sensor of claim 1 wherein the voltagecomprises an analog signal voltage.
 10. The magnetic field sensor ofclaim 1 wherein the linear magnetic field sensor and the interface areimplemented in a single integrated circuit.
 11. The magnetic fieldsensor of claim 10 wherein the single integrated circuit has only twoterminals, including the power supply terminal and the ground terminal.12. The magnetic field sensor of claim 6 wherein the voltage-controlledcurrent generating device further includes a resistor, and wherein theoperational amplifier, the transistor and the resistor are connected ina configuration that allows the operational amplifier to control thecurrent to remain proportional to the sensed magnetic field.